30 Precipitation reactions 14-16 Working in groupsSelf assessmentPeer assessmentSharing objectives and criteriaQuestioningUsing feedbackUsing tests

In this activity, students select diagrams that best represent the arrangement of ions before and after two solutions are mixed to form a precipitate. The diagrams focus thinking and discussion about precipitation reactions. They observe and interpret a practical demonstration and use ideas about particles to explain their observations.

Learning objectives

Students will be able to:

  1. explain and complete ionic equations to show how insoluble salts are made by precipitation.

Sequence of activities

Explain that you are going to mix together solutions of sodium chloride and silver nitrate which react together to produce insoluble silver chloride.

Issue a mini whiteboard to each student.

Ask:

  • students to draw diagrams with labels to predict what will happen
  • some students to show their ideas.

Demonstrate mixing sodium chloride solution and silver nitrate solution using large volumes of solution so that the formation of a precipitate is clear and dramatic.

  • Use questions to focus attention on the particles involved as a means of sharing the learning objectives with students.
Organise students into pairs.

Give one copy of Precipitation reaction diagrams to each pair and one Precipitation reaction sheet to each student.

Task students to:

  • choose a diagram that they think best represents the particles present in
    • the two separate solutions before mixing and
    • the solid and solution after mixing equal amounts of sodium chloride and silver nitrate solutions
  • record their choices on their Precipitation reaction sheet and explain why they made these choices.
Bring students to a plenary. Ask:
  • some students to describe and explain their choices
  • other students to add to these ideas
  • all students to write, on their mini whiteboard, an ionic equation to represent the mixing of solutions of sodium chloride and silver nitrate
  • students to show their equations
  • questions to reinforce the expected ionic equation
  • students to use these ideas to write down the ‘missing’ parts of the three ionic equations on their Precipitation reaction sheet
  • for answers and ensure that all students have a record of the expected answers
  • students to identify what they have learnt from the session and to complete that section on their Precipitation reaction sheet.
Take in the Precipitation reaction sheets and comment on the positive achievement shown by their explanations of their choice of diagram. Where appropriate, suggest how they might develop their understanding about precipitation reactions.

Assessment for learning commentary

The initial demonstration enables the teacher to vividly share the lesson objectives with students.

Students listen to the ideas of others and articulate their own ideas about precipitation reactions. This gives them a basis on which to judge what they have learnt. 

Relevant feedback will correct misunderstandings as well as helping the student to move forward.

Resources

    A mini whiteboard for each student
Download Word Download PDF A Precipitation reaction sheet for each student
Download Word Download PDF A Precipitation reaction diagrams sheet for each pair of students

Answers

I think that the particles in sodium chloride solution are best represented by diagram G
Diagram G contains sodium ions, chloride ions and water molecules.

I think that the particles in silver nitrate solution are best represented by diagram A
Diagram A contains silver ions, nitrate ions and water molecules.

I think that the particles in the solid, produced when equal amounts of sodium chloride and silver nitrate solutions are mixed, are best represented by diagram E
Diagram E contains silver ions and chloride ions in a lattice. The sodium and nitrate ions are still in solution.

I think that the particles in the solution, left behind when equal amounts of sodium chloride and silver nitrate solutions are mixed, are best represented by diagram L
Diagram L contains sodium ions, nitrate ions and water molecules. The silver and chloride ions have been removed as a solid.

1.

Pb2+(aq)

+

2I-(aq)

PbI2 (s)

2.

Ba2+(aq)

+

SO42-(aq)

BaSO4(s)

3.

Cu2+(aq)

+

OH-(aq)

Cu(OH)2(s)

Acknowledgment

K. Taber, Chemical misconceptions – prevention, diagnosis and cure, Volume 2: classroom resources, Precipitation. London: Royal Society of Chemistry, 2002.